Gas Dynamics Near and Far: Observational MHD of Interstellar Molecular Clouds and X-Ray Ablation of Asteroids for the Earth's Defense

Gas dynamics serves as a one of the foundational physical theories of astrophysics. In this dissertation, I will present my work on two different projects that touch on this subject.

In the first part, I will present my work concerning observational magnetohydrodynamics (MHD) in molecular clouds (MCs), those regions of the dense interstellar medium that form stars. MCs are lightly ionized and at large scales obey nearly ideal magnetohydrodynamics. Magnetic structure inherited from large scales influence the dynamics of the dense ISM, and can provide support against gravitational collapse that can slow or even halt star formation; however, the magnetic field is observationally difficult to measure. Polarized far IR/sub-mm dust emission - which arises due to alignment of dust grains with the local magnetic field - has emerged as a key tool to study magnetic fields in these regions. I will present my work on linking MHD simulations (which provide a complete 3D picture of a toy model MC) to submillimeter observations of MCs using synthetic polarimetric techniques. In the first chapter, the field of star formation is introduced and the physics of MHD outlined, as well as the physics of dust grain alignment and how that is linked to observationally accessible quantities. The second chapter reproduces the work presented in King et al. (2018), in which the first detailed statistical comparison between the BLASTPol observations of Vela C (Fissel et al. 2016) and numerical MHD simulations is conducted, under the assumptions of homogeneous grain alignment. In the third chapter, work is reproduced that has been submitted to MNRAS concerning the inclusion of heterogeneous alignment in synthetic observations. Then, in the fourth chapter, portions of work published in Chen et al. (2019) detailing Monte Carlo methods for studying polarization are presented.

In the second part I will present my work with the Planetary Defense group at LLNL, concerning the defense of Earth against impacting asteroids by ablating their surfaces with soft x-rays and disrupting them. Catastrophic impact events might be avoided by employing nuclear explosives as a means to disrupt or deflect an incoming asteroid. In the fifth chapter, the problem of planetary defense is introduced, as well as several means that have been proposed to mitigate the issue, including nuclear explosives. In this chapter, Smoothed-Particle Hydrodynamics as a numerical technique is also introduced. Lastly in the sixth chapter, work is presented that has been prepared for publication concerning the assessment of disruption strategies in planetary defense. A disruption event using nuclear explosives is modeled and used to motivate initial conditions for the general celestial mechanics problem of the fragment orbits. Using gravitational N-body techniques, a simulation suite is developed that is capable of assessing whether fragments still hit the Earth to a high degree of accuracy, leading to several basic results on disruption strategies and concluding with several hypothetical case studies.